In a textile yarn processing machine, particularly two-for-one twisters, it is desirable and has been the practice in some machines to monitor or sense the presence of yarns, threads, slivers, rovings, etc. (all collectively referred to hereinafter as "yarns") at certain monitoring locations for the purpose of effecting control functions, which may include the stopping or starting of the yarn processing machine or the particular work station of the machine, in the event of the absence or presence of yarn running through the particular monitoring location. While devices for such monitoring or sensing have previously been proposed, all such yarn monitoring devices exhibit various shortcomings to some degree.
In such textile yarn processing machines in which the running yarn forms a rotating balloon of yarn during such processing, a yarn guide eyelet is commonly positioned for limiting the upper end of the rotating balloon of yarn during passage of the running yarn through such eyelet. These yarn guide eyelets may be designed as a closed ring or a helical eye. As a consequence of centrifugal forces acting on the yarn by the rotating balloon of such yarn, tensions are created in the yarn in the zone of the rotating balloon of such yarn. Also, tensions are formed in the running yarn as a result of drawing off of the yarn from the supply package during the yarn processing operation. Both of the above produced tensions are added together or combined at the point in which the running yarn passes through the yarn guide eyelet and, therefore, produce combined forces on the eyelet in the running direction of the yarn which may be utilized for yarn monitoring or sensing purposes.
In a two-for-one twister textile yarn processing machine, as described in German Pat. No. 1,181,594, a helical type yarn guiding eyelet is maintained in generally horizontal position by these forces produced by the rotating yarn balloon when the yarn is running. In the event of yarn breakage or the absence of running yarn, the helical yarn guide eyelet swivels downwardly around a horizontal axis thereby triggering off various switching processes for spindle control mechanisms, including stop devices. The helical yarn guide eyelet is again supported by the rotating balloon of yarn after the yarn has been threaded in and the spindle assembly station reactivated. Thus, the helical yarn guide eyelet of this machine acts at the same time as a yarn sensor or monitor.
Yarn guide eyelets which can be swivelled or pivoted around fixed axes and which act at the same time as yarn sensors or monitors exhibit the drawback of the swivels or pivots being subjected to wear, mainly due to a high degree of pick-up of dirt, lint or other foreign matter and, thus, require continuous maintenance. The operation of this kind of pivoted or swivelly mounted yarn guide eyelets for acting as sensors or monitors for running yarn is not very reliable.
Yarn sensors or monitors normally act on electrical switches, more specifically micro-switches, which, inter-alia, operate spindle control mechanisms, including start and stop devices. These switches were heretofore positioned in the lever path of a holder or lever arm of a thread guide eyelet which was pivotally or swivelly mounted for engagement and disengagement thereby. However, micro-switches require normal switching forces of about 100 grams. Because of the arranging or positioning of such a switch within the swiveling or pivoting path of a thread guide eyelet lever arm, there was required long lever arms to produce the switching force normally required for the operation of such micro-switches. Therefore, these arrangements provided design deficiencies with respect to the lever arms for the pivotally or swivelly mounted yarn guide eyelet to also function as a yarn monitor or sensor.